Fiber optic cable is composed of a bundle of long, thin fibers of glass, plastic or other transparent material enclosed within a protective sheath. Modulated light pulses are transmitted through the fibers, which results in transmission of communications signals. Fiber optic cable is preferred over conventional cable such as copper cable because the fiber optic cable is lightweight, relatively thin, better preserves the signal characteristics, and carries a much higher rate and volume of information.
Fiber optic cable is more delicate and difficult to lay than conventional cable because it is relatively weak and brittle. Tension within the fibers will produce stresses, which in turn lead to fracturing and other physical modifications of the fibers. Such modifications of the fiber destroy or deteriorate the signal transmission characteristics of the fibers.
U.S. Pat. Nos. 4,468,003; 4,502,666; and 4,576,362 disclose systems for laying underground fiber optic cable. Some of the systems use capstans to assist in pulling the fiber optic cable through underground conduits and include means for maintaining the cable tension below a predetermined maximum. The environment of underground cable laying is significantly different from the environment of aerially stringing a cable, which normally involves mounting a reel of cable on a vehicle and unwinding the cable onto poles as the vehicle moves. Because the vehicle may accelerate at various rates, move at various velocities, go up and down terrain, turn corners, etc., the tension on the fiber optic cable may vary quickly and drastically. Consequently, the environment of aerially stringing fiber optic cable gives rise to problems different from those encountered in the environment of laying fiber optic cable underground.
One known method of aerially stringing fiber optic cable involves the use of strand support cable that has been previously suspended from a series of poles. First, several spaced pulley blocks are hung from the strand cable, then a tow line is fed through each of the blocks from the ending point to the beginning point where the fiber optic cable is to be run. An end of the fiber optic cable is attached to the end of the tow line at the beginning point of the run, and the tow line is drawn back through the series of spaced pulley blocks, towing the fiber optic cable behind. The tow line is pulled back to the ending point of the run, leaving the fiber optic cable strung through the blocks. Thereafter, a lashing machine is towed along the strand cable and fastens the fiber optic cable and the strand cable together while pushing the pulley blocks ahead for later removal. This method of stringing fiber optic cable moderates the magnitude of tensions within the fiber optic cable, but is very time consuming to perform and usually requires three or more people and two or more vehicles.
A second method of aerially stringing fiber optic cable also utilizes a preexisting strand cable. In this second method, fiber optic cable is unreeled from a vehicle up to a lashing machine which fastens the fiber optic cable and the strand cable together. This method is relatively fast and requires relatively few people and vehicles to perform the operation. Nevertheless, tension in the fiber optic cable can vary quickly and drastically as the vehicle moves. Also, if the vehicle cannot move close to the pole line to which the cable is being strung, then the cable cannot easily traverse from the reel mounted on the vehicle, and over to the lashing machine and the strand cable, thereby increasing the risk of cable damage. Moreover, when the vehicle slows or stops, the reel of fiber optic cable can continue to spin, which results in uncontrolled cable pay-out, which in turn can cause possible cable damage.
A third, very recent method of aerially stringing fiber optic cable involves inserting a fiber optic cable in a continuous plastic carrier tube and lashing the tube to strand support cable. This method incorporates the use of a reel handling trailer that is towed behind the cable laying vehicle. The reel trailer carries the plastic tube that is fed from the trailer to the rear of the cable laying vehicle, where a reel of fiber optic cable is mounted. The fiber optic cable and plastic tube are pulled together into a slitter-inserting tool whereby the plastic tube is slit open and the fiber optic cable is inserted inside the plastic tube. Then the plastic tube with fiber optic cable inside is lashed to a strand support. This method involves the use of a separate, bulky, expensive piece of equipment towed by the cable laying vehicle and requires additional set up time. This system greatly limits the maneuverability of the vehicle and sometimes inhibits access to pole lines. Additionally, since the fiber optic cable is free to slide inside the plastic tube, there exists the problem of long, steep, down hill runs, where the fiber optic cable (due mainly to vibration) eventually slides down inside the plastic tube. Such sliding creates high tension in the cable at the top of the run and also causes the fibers and/or cable to buckle due to column loading near the bottom of the run. This tension and buckling very easily causes micro-fracturing in the fibers, thereby greatly interfering with the transmission of light by the fibers.
A fourth method of aerially stringing fiber optic cable uses cable consisting of a strand support cable and an adjacent fiber optic cable molded together with a plastic sheath. Although the process of aerially stringing this cable is relatively easy and the cable will substantially eliminate damage to the fibers from longitudinal tension, any excessive bending or pressure contact of the cable will still damage the fibers. Moreover, the cost of the cable is extremely high due to the complicated fabrication of the cable. In addition, this self-support cable suffers from a major limitation in that when future expansion is warranted to provide additional communication services as the result of customer growth, new cable lines can not be lashed onto the self-support cable, unlike the more conventional method of direct lashing to strand support cable where additional cables can be easily lashed on top of each other. In many cases, existing self-support cables must be completely removed to relay higher capacity self-support cables, thereby substantially increasing the time and cost of expanding communication services.
In accordance with the description of the present invention herein, the term "laying cable" is deemed to include aerially stringing cable, laying cable in underground conduits, and other methods of extending a cable from one point to another point.